Evoluzione geologica tardo-pleistocenica ed olocenica del conoide complesso di Valle Majelama (Massiccio del Velino, Abruzzo)

Abstract

The Velino Massif (approx. 42° N) with the Majelama Valley alluvial fan developing on its southern flank, consists mainly of meso-cenozoic carbonatic sediments. The valley, dominated by peaks reaching altitudes of 2,100+2,200 m, shows several clear glacial evidences; at its mouth, at 1,100+1,050 m, there are remains of a morainic amphitheater attributable to the upper Pleistocene last glacial maximum. During the course of such period the Majelama Valley glacier could reach more than 6 km in length. In the upper part of the valley occur the remnants of the moraines deposited during the glacier's still-stands that took place on the Velino Massif during the stadial recessional phases known as Apennine I, II and III Stadials. The fan at the valley outlet is complex and incised by a number of small water courses which in their turn have formed alluvial fans. At present there is no water flowing in their beds, except in very limited stretches, during exceptional rainfall events. The Late Pleistocene evolution of the Majelama Valley fan has been reconstructed by means of morphological and stratigraphic studies, supported by a number 14C datings. It has been established that the main body of the fan is made up mainly of sediments of outwash fan origin, more recent than a sequence of depositional and erosive events after 31,220 ± 1,400 years B.P. and likely older than 14,580 ± 800 years B.P. 14C. They form a complex of five fans: the youngest of these is linked to the last of the three phases of the Apennine Stadial I. Then there are subsequent fluvial fans, the last two of which may date to Holocene times. In several quarries it has been observed that the outwash fan deposits cover a sequence consisting of: - fluvial sediments older than 39,500 years B.P. 14C; - pedogenized outwash fan deposits older than 33,140 ± 1,700 years B.P. 14C; - a paleosol buried in a period between 33,140 ± 1,700 and 31,220 ±1,400 years B.P. 14C; - a series of fluvial deposits and erosion surfaces more recent than the above mentioned paleosol. Fault-scarps, which affected the outwash fan deposits but do not appear to have influenced the evolution of the fan, being more recent than the main deposition phases, have been observed. On the whole the development of the fan seems to be linked to climatic and environmental causes. On the basis of the relationships between the sediments and the morphology and among the different fans, it has been possible to distinguish the following events: a) a rhexistasy phase: a period during which glacial bodies were established in the Majelama Valley. b) a biostasy phase: the soil developed and was buried between 33,140 ± 1,700 and 31,220 ± 1,400 years B.P. by 14C. For its characteristics, the soil could have developed during an Atlantic-temperate type climatic phase in presence of strong vegetal cover. c) a bioclimatic crisis; occurring as result of a cold and dry climatic change. d) a rhexistasy phase: datable between the burying of the soil and 14,580 ± 800 years B.P. with 14C method, during which the last glacial maximum occurred. This phase is characterized by three distinct periods: a first period (d1) corresponding to the establishment of glaciers in the upper valley; a second (d2) which could be of arid climate; a third period (d3) during which the climate fostered the maximum glacial expansion, e) a phase of prevalent biostasy: during this phase, the climate has been temperate and humid, but interrupted by relative arid phases or characterized by irregular rain fall events of strong intensity. Among the "d" rhexistasy phase and the "e" prevalent biostasy phase, a transition period took place.